But it would be fairly easy for me to create a C++ function you integrate at your end to add the keys. The code is readable in the vanitygen keyconv utility and was what I based my own JS client code on. At the C++ end it would require linking to the openssl library and only a few lines to add the keys. openssl provides the BigNum functions for addition. I'd have to look into.
Great - I am already using OpenSSL so that should be easy (am not using boost though so hopefully the code won't need it also).
I just checked the keyconv code again. It doesn't need boost. It's roughly a dozen lines of code that mostly use openssl fucntions,
EC_KEY_get0_private_key
EC_GROUP_get_order
BN_mod_add
and a few init/free type openssl calls.
So it should be straightforward. Once I've got further, assuming I actually go live with this, and have some time, I can write up a function and test it out. Curl should work fine for actually accessing the api. Would you be getting generating keys single or in batches? Batches is far more efficient unless the prefix is long (6-7+ chars).
What happens if I have a vanity address and your site goes down?
Once you have the vanity address (and key) my site wouldn't matter at all. They are normal addresses just as if you generated one yourself. Requesting addresses in a batch would make sense as you wouldn't depend on the site to create an address on demand. The api would allow you to request a batch and then call back later to retrieve the results. For short prefixes this can be quite fast. 100 x 4 chars ones in a minute or so is reasonable. A single short prefix address would take a 5-15 seconds due to overhead and polling time on the GPU backend.